The function of a voltage-controlled oscillator is to generate a periodic output signal, the frequency of which is a function of a continuous voltage applied to an input. It thus allows periodic signals to be generated, the frequency of which is open to selection.
There are several types of voltage-controlled oscillator. A first example of a VCO is the VCO of the RC type, which is based on loading and unloading a capacitance through a capacitor.
A second example of a VCO is the voltage-controlled differential oscillator, designated hereinafter as a differential VCO. The differential VCO conventionally includes four transistor based differential pairs.
FIG. 1 shows a simplified diagram of a conventional differential VCO 10. The differential VCO includes first 11, second 12, third 13 and fourth 14 amplifying stages. Amplifying stages 11 to 14 are polarised by means of a bias voltage VBIAS applied to an input 15 of the differential VCO 10. Each stage includes two inputs, 111 and 112, 121 and 122, 131 and 132, 141 and 142 respectively, and an output 133, 123, 133 and 143 respectively.
The output 133 of the third stage supplies the periodic output voltage of the differential VCO 10 the frequency of which depends on the bias voltage VBIAS.
FIG. 2A shows the diagram of a conventional amplifying stage of the differential VCO 10, for example the first stage 11. For example, all the stages 11 to 14 of the differential VCO 10 are identical.
This first stage 11 conventionally includes:                a conventional differential pair 114 based on two P-type MOS transistors M1, M2;        a current source 115 including one P-type MOS transistor M3; and        an active load 116 based on two N-type MOS transistors M4, M5.        
The inputs 111, 112 of the first stage 11 are connected to the gates of the transistors M1, M2 of the differential pair 114. The output 113 of the first stage 11 is connected to the drain of the transistor M2 of the differential pair 114.
The sources of the transistors M1, M2 of the differential pair 114 are connected to one and the same common node N.
A known technique (shown in FIG. 2B) for increasing the power supply rejection of an amplifying stage 21 is to connect the NWELL wells of the transistors 201, 202 of the differential pair 214 to a common node N.
When designing a differential pair that includes two transistors M1 and M2, each of the transistors of the pair, for example the transistor M1, can conventionally be replaced by two transistors M11, M12 each having a gate width two times smaller and a gate length identical to that of the transistor M1.
FIG. 3 shows the diagram of a differential pair 314 in which each of the transistors has been split so as to form two pairs of transistors M11 and M12, M22 and M21 respectively. FIG. 4 is a diagram of the topology of the transistors M11, M12, M22 and M21 of the differential pair 314 in FIG. 3 in the case of a “crossed pair” arrangement. A number of metal strips (not shown in FIG. 4) allow the electrical connections in FIG. 3 to be made between the different contacts of the transistors.
It may be noted that in this FIG. 4 but also in those that follow, the areas marked by black dots are doped zones in which the transistors are made.
The 4 transistors of the differential pair 314 are equivalent to two transistors forming the pair 314. The “crossed pair” arrangement makes it possible to ensure a good uniformity of dimensions of these two equivalent transistors when the pair is manufactured.
Indeed, any variation in dimension due to the manufacturing process, and any interference that causes parasitic effects in the transistors (noise, temperature variation, etc) is transmitted in the same way to each of these two equivalent transistors.
In fact, a lack of uniformity in respect of the dimensions of the transistors of a differential pair (for example the pair 314 in FIG. 3) generates an offset voltage between the two inputs of the pair (311 and 312) that may well interfere with the operation of an amplifying stage that includes the pair.
Specialists in the manufacture of transistors therefore consider that it is necessary to employ the “crossed pairs” arrangement when manufacturing a differential pair.
Conventionally, in a differential pair arranged as a “crossed pair”, the man skilled in the art combines the four transistors in one and the same well 41 (in FIG. 4). In this way, when a VCO is made based on conventional differential pairs of this kind, each of the pairs is included in a different well.
Unfortunately, a first drawback of this conventional arrangement is that it generates parasitic capacitances (capacitances due to the metal strips connecting the transistors, diffusion capacitance of the transistors, bottom or sidewall capacitance between the wells of the transistors and the substrate, etc) that restrict the performance of the differential VCO, in particular by reducing its oscillation frequency.
VCOs are able to operate in different frequency ranges and to be made according to different technologies, each associated with a pitch that is generally indicated in microns.
The growing need to reduce power consumption and the dimensions of electronic systems based on integrated circuits has led designers of VCOs of this kind to use manufacturing technologies that have an increasingly small pitch.
In this way, for example in order to make a phase locked loop (or PLL) for a USB port according to the USB 2.0 standard, it may be necessary to make VCOs by means of technologies that have a pitch below 0.5 μm and which operate at a frequency of about 480 MHz (for an internal clock of 12 MHz).
One drawback of differential VCOs is, according to the man skilled in the art, that they are unable to operate at significant frequencies (above 300 MHz, in very small-scale technologies, in other words below 0.5 μm).
Indeed, the person skilled in the art considers that, should it be required to make a differential VCO operating at these frequencies from a technology below 0.5 μm, the transistors of the differential pairs of this differential VCO would be too small to ensure good uniformity in the dimensions of the transistors of the VCO despite the use of the “crossed pairs” arrangement.
It is for this reason, generally speaking, that VCOs operating at high frequencies (above 300 MHz) and made from very small-scale technologies (below 0.5 μm) are VCOs of the RC type.